"The diagnosis of drowning is one of the most difficult in forensic pathology. Drowning is death through the aspiration of fluid into the air passages. Signs of immersion only demonstrate submersion of the body for a period of time but are not signs of drowning. The best signs of drowning are froth around the mouth and nostrils and lung distension. Lung histology in drowning victims shows non-specific lesions such as "emphysema aquosum" and alveolar edema........The diagnosis of drowning is based on police investigations, forensic autopsy, microscopic analysis, and biochemical tests, but never solely on pathology findings."
http://www.ncbi.nlm.nih.gov/pubmed/14752383
I did not know that a diagnosis of drowning was such a difficult one! I hope if/when the pathologists who made the original cause of death determination testify in this trial that they "man up" (or "woman up") to the fact that they were likely incorrect in their determinations.
There are some other newer, very sophisticated computer measurement based methods used for examining the cells of the alveoli - which are the specialty cells within the lung where oxygen and carbon dioxide are passed into and out of the bloodstream. They actually look at the shape of the cells, which become distorted during a drowning. Plus there is hemorrhaging within the lung which is what produces the bloody froth.
It does sound like Dr. Perper is correct, that she drowned.